Actuators producing mechanical movement of an object in response to the application of a stimulus, such as electrical power, are well known. Among actuators that are small and that respond to the application of electrical power to produce motion are electrothermal actuators. Examples of such actuators are described in U.S. Pat. Nos. 4,759,189, 4,887,429, and 7,536,860, each of which is incorporated herein by reference. Within these electrothermal actuators, a closed chamber contains a working fluid. The working fluid is mostly a liquid at ambient temperature and evaporates to the gas phase when heated. That gas phase of the fluid expands upon further heating, increasing internal pressure within the chamber. In the following description, the reference to a fluid encompasses both the liquid and gas phases, with the gas phase expanding upon heating to provide the motive force of the actuator. The chamber includes an electrically powered heater that supplies heat to the fluid, in response to an electrical current supplied to the heater. The heat produces a phase change in the working fluid and a pressure increase within the chamber. In response to the increased internal pressure in the chamber, a flexible rolling diaphragm, usually peripherally clamped to the housing of the electrothermal actuator, is displaced. The diaphragm displacement pushes a piston that may drive a piston rod in a linear direction, for example, to increase protrusion of the piston rod from the housing of the electrothermal actuator.
Typically, an electrothermal actuator includes a return spring biasing the piston toward the position before heating of the working fluid. The expansion of the working fluid and the movement of the diaphragm provide a force that counteracts the restoring force of the spring. In order to maintain a particular extension of the piston rod in many conventional electrothermal actuators, the pressure within the chamber must be maintained by continuing the flow of electrical current to the heater. In some applications, the continuing consumption of electrical power to keep the piston rod extended is undesirable. Elimination of or at least a reduction in the amount of electrical power required to maintain the extension of the piston rod is, in many applications, highly desirable.
In addition, in actuators in which the working fluid remains in the gas phase during the entire time a piston rod is extended, the response time for retraction of the piston rod can be undesirably long. Once electrical current to the heater is stopped, the working fluid must cool for the pressure in the chamber to decline, permitting retraction of the piston. A much shorter response time in retraction of the piston rod is very important in some applications of electrothermal actuators.
One known electrothermal actuator, described in U.S. Pat. No. 5,396,770, reduces, and even eliminates, the necessity for continuing the supply of electrical power after extension of the piston rod, for maintenance of the piston rod in an extended position. The electrothermal actuator described in that patent includes a mechanical latch mechanism that engages the piston rod when the piston rod is fully extended. The latch prevents retraction of the piston rod so that electrical current flow to the heater can be terminated. In order to release the piston rod so that it retracts, the latch mechanism must be released. The latch mechanism includes a coil and a magnetic member. A magnetic field is produced by applying an electrical current to the coil. The magnetic field moves the magnetic member, releasing the latch, so that the piston rod retracts. When the piston rod is latched, the piston rod does not return to the initial position, with the piston rod retracted, upon loss or withdrawal of all electrical power. In other words, the fail-safe position of the piston rod depends upon whether the piston rod has been latched before the loss of power. In many applications it is desirable that the fail-safe position, as in actuators without mechanical latches, be the retracted position of the piston rod. In this mechanically latching actuator, piston rod retraction time can be shortened if the latch is not released until after the working fluid has cooled.
U.S. Pat. No. 7,536,860 describes an actuator in which a magnetic field is used to hold the piston rod in an extended position. In one described embodiment, the piston rod carries a magnetic collar. The housing of the actuator includes an electromagnet positioned at the end of the housing where the piston rod is located. When the electromagnet is energized and the magnetic collar is nearby, the piston rod can be maintained in the extended position with no electrical power applied to the heater. The only power consumption is by the electromagnet to retain the piston rod in the extended position. Provided the working fluid has cooled while the piston is held in the extended position, the piston rod can retract relatively quickly upon termination of current flow to the electromagnet.